1. Field of Use
This invention relates generally to high voltage current-limiting cartridge-type electrical fuses.
In particular, it relates to improved means for terminating the ends of fusible elements, such as ribbons or wires, employed therein.
2. Description of the Prior Art
A high voltage current-limiting cartridge-type fuse typically comprises a hollow cylindrical insulating housing; electrically conductive end cap assemblies secured to opposite ends of the housing; a multi-ribbed dielectric spider assembly having electrically conductive metallic terminator plates connected at opposite ends disposed within the hollow housing with each terminator plate electrically and mechanically connected to an associated end cap assembly; a plurality of fusible ribbons helically wound on the spider assembly with an end of each ribbon electrically and mechanically secured to an associated terminator plate; an axially movable visual indicator pin mounted on one end of the spider assembly and movable by a biasing spring from a retracted position wherein it is secured by a fusible wire to a tripped (fuse-blown) position wherein it projects through aligned holes in an associated terminator plate and end cap assembly; and granular dielectric material, such as sand, filling the housing. U.S. Pat. No. 4,220,940 assigned to the same assignee as the present application illustrates a prior art fuse of the aforesaid character.
When a fuse of the aforesaid character is subjected to an electric overload in the circuit in which it is employed, the fusible element vaporizes, accompanied by temperatures on the order of 2000.degree. F. occurring within the hollow housing, and arcing occurs between the points at which the fusible element was terminated. If a fusible element is terminated directly on a relatively thin flat terminator plate, there is a danger that the terminator plate and its associated end cap assembly can burn through. If the end cap assembly burns through, the desired gas pressurization within the sealed fuse housing which aids in arc extinction (and arises in part from fusion of the granular dielectric within the housing) is lost and arcing may continue between the terminator plates and/or cap assemblies at opposite ends of the housing.
In order to overcome this problem it is the practice to employ a construction wherein an end of a fusible element is terminated ahead of the terminator plate (i.e., in a direction axially inwardly and away from the terminator plate and its associated cap assembly) and on a component or arcing electrode of relatively great mass electrically and mechanically affixed to the terminator plate. In some prior art fuses such a component takes the form of a metal block or terminator finger affixed to a terminator plate on the side thereof facing away from the associated cap assembly and to which the fusible element is welded. As indicated above, the reason for this is at certain levels of fault current the fuse elements require relatively long arcing times to clear. If the arcing burns back the element to the termination finger, it is desirable that the mass of the finger prevent the arc from burning through to the cap assembly before the fault is cleared. If the arc does reach the cap assembly, the possibility exists that it will burn through the cap assembly before the fuse clears. If this happens, the fuse may never clear because the unsealed fuse allows gas pressure to rapidly escape which encourages the ionized gases to follow a path perpendicular to the element windings. Because of the high voltage between element turns, they can flash-over resulting in failure of the fuse to clear. While such prior art solutions result in fuses having desirable electrical properties, the solutions create fuse assembly and production problems in that welding of the fusible element (whether ribbon or wire) becomes to an arcing electrode difficult, time-consuming and costly. When attempting to resistance-weld fuse elements to the arcing electrodes (as opposed to soldering), it is difficult to physically get the welding tool electrodes into the tight configuration of the arcing electrodes because the adjacent fusible elements block the access to the next arcing electrodes. Also, the spider blocks access to the backside of the arcing electrodes. In welding it is necessary to have an electrode on each side of the termination to supply both electrical contact and pressure on the two pieces (fuse element and arcing electrode) to be welded.